1. Field of the Invention
The present invention relates to a MEMS-type high-sensitivity inertial sensor and to the manufacturing process thereof.
2. Description of the Related Art
As is known, techniques of micromachining of semiconductors are used also for manufacturing electromechanical microdevices (so-called micro-electro-mechanical-systems or MEMS), such as sensors and actuators of various types. In particular, the techniques of micromachining are advantageously used for manufacturing inertial sensors, utilized for example in the automotive sector or within apparatuses equipped with stand-by functions, for recovery of functionality starting from the stand-by condition upon detection of a movement.
Currently, inertial sensors are formed preferably by surface micromachining, wherein the mobile and fixed elements that form the sensor (rotor and stator and corresponding electrodes) are formed in a same structural layer, typically a semiconductor layer, of a mono-crystalline or polycrystalline type.
In this type of sensors, the thickness of the structural layer influences both the compliance of the structure to mechanical stresses (stiffness) and the mass. Any increase in the thickness of the structural layer brings about an increase in inertia (as a result of the increase in the mass), and consequently in the mechanical sensitivity of the sensor, i.e., the capacity for the rotor to modify its relative position when subjected to a stress, without any increase in the overall dimensions of the sensor.
However, the increase in the thickness determines an increase not only in the mass of the system but also in the stiffness of the springs, thus countering the improvement in the sensitivity of the sensor.
Not even other ways of increasing the mechanical sensitivity of the sensor are able to solve the problem. For example, by making springs that are more compliant, it is possible to increase the degree of movement of the rotor for a same applied stress; however, in this case the capacity to reject movements in other directions is reduced, and the sensor could yield false readings.
For a better understanding of the problem referred to above, in particular the one linked to the increase in thickness, reference may be made to FIGS. 1-3, corresponding to a known translational inertial sensor.
In FIGS. 1 and 2, an inertial sensor 1 comprises a body of semiconductor material, formed by a substrate 2 and by a structural layer 3, overlying one another. The structural layer 3 forms a rotor 5 and a stator 6, extending above the substrate 2 but set at a distance therefrom by an air gap 7. The air gap 7 is obtained in a known way by removing a portion of a sacrificial layer, for example of silicon oxide, a remaining portion of which is designated by 8.
The rotor 5 comprises a suspended mass 10, here of a substantially parallelepipedal shape, and first and second mobile electrodes 11a, 11b extending from two opposite sides of the suspended mass 10 and arranged parallel to one another. The stator 6 comprises first and second fixed electrodes 12a, 12b extending parallel to one another and to the mobile electrodes 11a, 11 b. In particular, the first fixed electrodes 12a are comb-fingered to the first mobile electrodes 11a, and the second fixed electrodes 12b are comb-fingered to the second mobile electrodes 11b. The fixed electrodes 12a, 12b extend from a fixed supporting structure 13, carried by the substrate 2, only some parts whereof are visible in FIGS. 1 and 2.
As represented schematically in FIG. 1, the rotor 5 is supported by the fixed structure 13 via elastic elements or springs 15 that enable oscillation of the rotor 5 in the direction indicated by the arrow A. The anchorage portions of the rotor 5 are obviously electrically insulated from the stator 6 via junction insulations, dielectric insulations, or by trenches, in a per se known manner that is not illustrated in the drawings.
In use, the rotor 5 is biased at a sinusoidal a.c. voltage V1, as represented in FIG. 1 by a voltage generator 16, while the stator 6 is connected to a sensing circuit 20 comprising two operational amplifiers 21a, 21b, each connected to a respective set of fixed electrodes 12a, 12b. 
In detail, the operational amplifiers 21a, 21 b have an inverting input connected to the respective set of fixed electrodes 12a, 12b, and a non-inverting input connected to ground. A feedback capacitor 23a, 23b is moreover connected between an output 24a, 24b and the inverting input of a respective operational amplifier 21a, 21b. The resulting electrical diagram is illustrated in FIG. 3, which relates to both the operational amplifiers 21a, 21b and wherein consequently the elements represented have been identified without using the letters a and b.
As may be noted in particular from FIG. 2, the fixed electrodes 12 and the mobile electrodes 11 have a thickness t equal to the thickness of the structural layer 3 and a length l, and are arranged at a distance from the adjacent electrode of an opposite type (i.e., mobile or fixed) by a space g, which is variable and depends upon the instantaneous position of the rotor 5.
In practice, as illustrated in the equivalent circuit of FIG. 3, the fixed electrodes 12 and the mobile electrodes 11 on each side of the suspended mass 5 form a variable capacitor 25 having a capacitance C1 given by:
                    C1        =                                            ɛ              0                        ⁢            N            ⁢                                                  ⁢                          A              g                                =                                    ɛ              0                        ⁢            Nl            ⁢                                                  ⁢                          t              g                                                          (        1        )            where ε0 is the dielectric constant in a vacuum; N is the number of fixed electrodes 12 connected to each operational amplifier 21; l, t, g are the quantities indicated above; A represents the facing area, which here is approximately equal to l×t, since the length of facing between fixed electrodes 12 and mobile electrodes 11 can be considered, to a first approximation, equal to l.
From Eq. (1) it is evident how the capacitance C1 is directly proportional to the thickness t of the structural layer 3.
In general, it is moreover possible to state that the mass M of the rotor 5 and hence substantially of the suspended mass 10 is given by the formula:M∝ρtMA=ρtMltyp,rot2  (2)where ρ is the density of the material (silicon), tM is the thickness of the suspended mass 10, and ltyp,rot is the typical length (which is linked to the width of the suspended mass 10 and thus to the overall dimensions) of the sensor 1.
The stiffness k of a spring, instead, is given by:
                    k        ∝                              t            k                                l                          typ              ,              s                        n                                              (        3        )            where tk is the thickness of the spring, ltyp,s is the typical length of the spring, and n is a coefficient linked to the type of sensor and is typically equal to 3 for planar sensors, whether of a linear type or of a rotational type.
The sensitivity S of a sensor of this type is:
                              S          ∝                      M            k                          =                  ρ          ⁢                                          ⁢                                    t              M                                      t              k                                ⁢                                    l                                                typ                  .                                ,                rot                            2                        ·                          l                              typ                ,                s                            n                                                          (        4        )            From Eq. (4) it may thus be noted that, in a typical micromachining process, in which the two thicknesses tM and tk are equal, the sensitivity S is invariant to the variation in thickness.
Thus, currently, when it is desired to increase the sensitivity of the sensor, the design aims at increasing the occupation of area (i.e., ltyp) of the sensor either to increase the mass of the system or to reduce the stiffness of the elastic suspension springs.
Similar considerations apply to an inertial sensor of rotational type, the simplified structure whereof is shown in FIG. 4 where, for reasons of clarity of illustration, the same reference numbers as those of FIGS. 1 and 2 have been used. In detail, FIG. 4 shows an inertial sensor 1′ having a rotor 5, a stator 6, a suspended mass 10, mobile electrodes 11, fixed electrodes 12, springs 15, and a fixed structure 13. In a way not shown, the inertial sensor 1′ is connected to a sensing circuit similar to the sensing circuit 20 of FIG. 1, so that the inertial sensor 1′ has the equivalent circuit illustrated in FIG. 3 and has an output voltage V0 given by Eq. (4).
In this case, the inertial sensor 1′ has a moment of inertia Jz with respect to the axis Z of rotation equal to:
                              J          Z                =                              1            2                    ⁢                      MR            2                                              (        5        )            where M is the mass (practically coinciding with that of the suspended mass 10), and R is the mean radius of the rotor 5, substantially due to the radius of the suspended mass 10.
As may be noted, the moment of inertia is directly proportional to the mass, which is in turn directly proportional to the thickness. Since the mechanical sensitivity of the inertial sensor 1′ of rotational type is linked directly to the moment of inertia, the increase in the thickness of the structural layer accommodating both the rotor 5 and the stator 6 determines an increase in the mechanical sensitivity. However, also in this case this effect is nullified at the sensing circuit. As for the inertial sensor 1 of FIGS. 1 and 2, then, it is not possible to increase the sensitivity of the inertial sensor simply by increasing the thickness of the structural layer.